Numerous insects are vectors for disease. Mosquitoes in the genus Anopheles are the principle vectors of malaria, a disease caused by protozoa in the genus Trypanosoma. Aedes aegypti is the main vector of the viruses that cause Yellow fever and Dengue. Other viruses, the causal agents of various types of encephalitis, are also carried by Aedes spp. mosquitoes. Wuchereria bancrofti and Brugia malayi, parasitic roundworms that cause filariasis, are usually spread by mosquitoes in the genera Culex, Mansonia, and Anopheles. 
Horse flies and deer flies may transmit the bacterial pathogens of tularemia (Pasteurella tularensis) and anthrax (Bacillus anthracis), as well as a parasitic roundworm (Loa loa) that causes loiasis in tropical Africa.
Eye gnats in the genus Hippelates can carry the spirochaete pathogen that causes yaws (Treponema pertenue), and may also spread conjunctivitis (pinkeye). Tsetse flies in the genus Glossina transmit the protozoan pathogens that cause African sleeping sickness (Trypanosoma gambiense and T. rhodesiense). Sand flies in the genus Phlebotomus are vectors of a bacterium (Bartonella bacilliformis) that causes Carrion's disease (oroyo fever) in South America. In parts of Asia and North Africa, they spread a viral agent that causes sand fly fever (pappataci fever) as well as protozoan pathogens (Leishmania spp.) that cause Leishmaniasis.
Most blood feeding insects, including mosquitoes, sandflies, Testse flies, use olfactory cues to identify human hosts. This group of hematophagous insects can transmit a wide assortment of deadly human diseases that together cause more suffering and deaths globally than any other disease condition. Diseases transmitted by such insects include malaria, dengue fever, yellow fever, West Nile virus, filariasis, river blindness, epidemic polyarthritis, Leshmaniasis, trypanosomiasis, Japanese encephalitis, St. Louis Encephalitis amongst others.
The olfactory system can detect and discriminate amongst an extremely large number of volatile compounds in the environment, and this is critical for important behaviors like finding hosts, finding food, finding mates, and avoiding predators. To detect this wide variety of volatiles, most organisms have evolved extremely large families of receptor genes that typically encode 7-transmembrane proteins expressed in the olfactory neurons. Little is known, however, about what structural characteristics of small volatile molecules are important for behavior modification. The predicted odors provided herein are able to manipulate the olfactory-based behavior of an organism by making use of computationally identified important structural characteristics.
Volatile chemical space is immense. Odors in the environment that have been catalogued in some plant sources alone number more than a couple thousand. A very small proportion of chemical space has been systematically tested for the ability to modify behavior, and a very small fraction of odor receptors, whose sequences are known, have been tested for their ability to be affected by behavior modifying odors. The complete 3-D structures of odor receptor proteins have not yet been determined, thus modeling of odor-protein interactions is not yet possible except in rare instances. Furthermore, were a 3-D receptor structure to become available, application of one odor-receptor interaction to study others may be confounded by the possibility of multiple ligand binding sites in a single receptor, as well as the sequence divergence amongst different odor receptors. The disclosure was identified by intelligent and rapid screening of untested volatile chemical space through computational identification of important characteristics shared between known behavior modifying compounds, circumventing many of the previously described obstacles. Additionally, one can screen potential odors for toxicological safety. The disclosure has been used to identify molecular features important in mosquito avoidance. The identified features were then used to screen a vast chemical space, predicting odors that interrupt host-seeking behavior.
Several repellent compounds have been identified to date. These compounds range from naturally occurring extracts to commercially manufactured compounds. The degree of protection, duration of protection, and safety of these odors varies greatly. The gold standard of these compounds generally considered DEET.
DEET (N,N-diethyl-3-methylbenzamide) has been used for insect repellency for over 50 years. Protection is generally provided by direct application to the skin in concentrations ranging from 3 to 100 percent (Household products database of NLM). While results vary across experiments, DEET has been shown to act as an irritant and in some cases may cause skin reactions. In a recent study DEET has also just recently been shown to inhibit acetylcholoinesterase in humans, which is an important neurotransmitter. DEET is also known to dissolve several products including certain plastics, synthetic fabrics, painted or varnished surfaces. How DEET is detected by arthropods is currently unknown. Several candidate methods have been proposed, but sufficient evidence that any of these methods is the direct avoidance-inducing pathway has not been demonstrated. As an example, it has been demonstrated that Culex quinquefasciatus are able to directly detect DEET through a short trichoid sensillum in a dose dependent manner. It has also been proposed that Drosophila are able to detect DEEN through gustatory receptors. It is possible that mosquitoes recognize this compound through a combination of olfactory and gustatory pathways.
Several other terpenoid compounds with repellent properties including thujone, eucalyptol, and linalool have also been identified. These compounds were shown to directly activate a trichoid sensillum housed odor receptor, which is also activated by DEET, only more strongly than DEET itself.
Icaridin, which is also called picaridine, is also used as an insect repellent. Similarly to DEET it acts as a repellent to several different insect species. Icaridin has the added benefit of not melting plastics. It has been found to be as effective as DEET at repelling insects, while being less irritating than DEET.
In two recent studies, 34 N-acylpiperdine and 38 carboxamide mosquito repellent candidates were synthesized and compared for their effectiveness. 19 of the N-acylpiperdine and 7 of the carboxamide compounds were either as effective as or more effective than DEET at repelling Aedes aegypti at a concentration of 25 μmol/cm2 using a protection time assay. The mode of repellency for all of these compounds is unknown. As the N-acylpiperdines and some of the carboxamides are larger and likely have a lower vapor pressure than DEET, it is possible that the increased protection times of these compounds is due to a slower evaporation rate.
Traditional vector control methods often involve the heavy use of chemical insecticides that are harmful to the environment and often to human health. Moreover, insects can develop resistance to these chemicals, suggesting that there is a need to identify novel ways of insect control that are effective, cheap, and environmentally friendly. Integrating methods that inhibit vector-human contact, such as vector control and the use of insect repellents, bednets, or traps, may play a complementary and critical role in controlling the spread of these deadly diseases.
In insects host-odor cues, among others, are detected by olfactory receptor neurons (ORNs) that are present on the surface of at least two types of olfactory organs, the antennae and the maxillary palps. The antenna is the main olfactory organ and its surface is covered by hundreds of sensilla, each of which is innervated by the dendrites of 1-5 ORNs. Odor molecules pass through pores on the surface of sensilla and activate odor receptor proteins present on the dendritic membranes of the ORNs.
The odor receptor (Or) gene family in insects was first identified in D. melanogaster. It includes a highly divergent family of 60 Odor receptor (Or) genes that encode proteins predicted to contain seven trans-membrane regions.
Odor responses of ORNs on the surface of the antennae and maxillary palps have been studied using two separate techniques. Whole organ recordings called electroantennograms (EAGs) and electropalpograms (EPGs) have been used to detect the aggregate electrical activities from a large number of neurons in response to odors. A more sensitive and exact method has also been used to examine the functional properties of olfactory neurons within a single sensillum, and neurons that respond to behaviourally important ligands such as CO2, ammonia, phenols, 1-octen-3-ol, lactic acid, and carboxylic acids have been identified.
Odor receptor responses to odorants have been tested in vivo in the organism of interest predominately through two separate techniques. One approach involves whole organ recordings called electroantennograms (EAGs), eletropalpograms (EPGs), and electroolfactograms (EOGs) which have been used to detect the aggregate electrical activities from a large number of olfactory neurons in response to odors. This technique does not allow for differentiation between odor receptor neuron responses and thus does not allow for identification of individual odor receptor responses to an odorant. A more sensitive and precise technique called single unit electrophysiology allows for individual odor receptor neuron responses to odors to be quantitatively measured. This technique either requires the odor receptor map to have been previously established by molecular tools or use of an “empty-neuron” system that utilizes a transgenic approach.
Additionally, other in vivo techniques have been used involving testing individual odor receptors of interest through transgenic expression in other organisms. Heterologous expression of Odor receptor genes from many species has been performed in Xenopus oocytes and Human Embryonic Kidney (HEK) 293 cells. Exposure of these cells to volatile compounds allows for a quantitative measure of response.
While these systems do provide a means to specifically express an odor receptor and obtain a quantitative measure of activation to a panel of odorants, their use is a very time consuming, expensive, and difficult process. Use of the “empty neuron” system and other heterologous expression approaches require transgenic fly lines to be produced or cDNA expression constructs made for each odor receptor to be tested. It has also been debated whether these expression systems produce wild type responses in all cases, as some cell specific components such as odorant binding proteins (OBPs) may be absent. Additionally all systems require the requirement of purchasing odors, diluting them, and performing the technically challenging testing of odorants.
In previous studies, individual odor receptors have sometimes been found to recognize compounds of similar functional groups containing similar hydrocarbon chain lengths. In addition it has also been shown that many Ors can be responsive to multiple distinct groups of structurally similar compounds. This property of odor receptors recognizing structurally similar compounds provides a framework for using cheminformatic similarity measures to predict novel active odorants.
Molecular descriptors are able to describe the structure of molecules through computationally derived values, which represent zero, one, two, or three-dimensional information of a compound. These descriptor type dimensionalities confer molecular information through classes such as constitutional, structural fragment, topographic, or spatial information, respectively.
Comparison of molecular descriptors to identify commonalities between highly active odorant structures has recently proven to be highly beneficial. In species where a specific behaviour, such as avoidance, has been tested against a panel of odors it is possible to use molecular descriptors to identify novel potential ligands using the known actives as a training set. For instance, the structure of N,N-diethyl-m-toluamide (DEET) was recently used to create a focused structural library, which was computationally ranked using Artificial Neural Networks (ANNs), and used to identify a more potent mosquito repellent. In another study a group analyzed Drosophila ORN responses to odors to identify activation metrics that were used to predict and test ligands from a small set of 21 compounds (Schmuker et al., 2007). The success rate of this strategy, as established by applying a neuronal firing rate cut-off of 50 spikes/sec to categorize activators, was <25%. Most recently a multi species approach was used to identify molecular descriptors that were important in compounds involved in olfaction however predictions were not possible. In another study by the same lab, an electronic nose was trained such that when presented with a novel odor it could predict whether or not the odor would activate an individual Or.
Therefore, there remains a need for computational methods that identify molecular descriptors that are useful in identifying arthropod repellants and/or attractants.